Gas turbine engine components such as rotating blades and stationary vanes are subjected to high temperatures and stresses for extended periods of time. Additionally, there are high thermally induced stresses and strains associated with the cycling of a turbine engine between shutdown and power operating conditions. Materials with a low elastic modulus are preferred for minimizing the thermal stresses associated with engine cycling, thereby providing extended component life.
Cast columnar grained and single crystal materials have been developed to take advantage of high creep strength along the growth axis (001) of the material. These materials also exhibit crystallographic anisotropy. Columnar grained materials have one lowest modulus orientation; i.e. longitudinal, aligned with (001). Single crystal materials possess cubic symmetry and thus have three lowest modulus orientations—(001), (010) and (100). Thus, gas turbine components are ideally manufactured as single crystals wherein the grain orientation is controlled within the component to align the low modulus directions with the axes of highest thermal strains. However, there are many manufacturing challenges in growing a single crystal in the form of a complex turbine component, and therefore complex castings are usually of low casting yield and thus are very costly. Columnar grained materials are a compromise between the properties of equiaxed (random grained) and single crystal materials. While these materials provide optimum mechanical properties in only one direction, they present fewer manufacturing challenges and are generally of higher casting yield and lower cost than single crystal materials, and they are therefore preferred for certain gas turbine applications.
U.S. Pat. No. 4,464,094 describes a gas turbine engine vane incorporating an airfoil made of a columnar grained material and shrouds made of a single crystal material. The airfoil and shroud components are separately cast superalloy materials that are subsequently joined by welding. The resulting vane component exhibits material orientations that are advantageous relative to the direction of peak stresses in the component, and it exposes a minimum number of grain ends to hot combustion gasses during operation of the turbine component. Proper spatial orientation of the shrouds and airfoil is ensured by the use of temporary spacer bars to interconnect the opposed shrouds during welding of the airfoil to the shrouds. While the resulting product may be preferred over previous designs utilizing only columnar grained materials, the cost of manufacturing and the risk of fabrication defects are increased by the necessary welding operation.